“On the face of it, cities seem to be made up of two very different things: a slowly changing, physical system, made up of of buildings linked by streets, roads and infrastructure; and a more rapidly changing human system made up of movement, interaction and activity. The human system seems to be superimposed on the physical system, and in some way to constitute its functional system, but has an unknown relation to it. The question every theory must address is: what, if any, is the relationship between the two systems ?”